It is estimated that up to 80% of the dust particles illuminated by incident sunlight and made visible to the naked eye in a domestic environment are derived from skin. In a warm environment, dust mites feed on skin-derived dust particles, breaking it down by using proteases in their digestive system. Such proteases are found in not insignificant levels in dust mite faeces, and it is now established that it is excreted proteases which act as allergens to individuals who are liable to have an allergic response to house dust. Concentrations of excreted protease are found in relatively high levels in carpets, bedding, pillows and mattresses, all of which provide a suitable environment for dust mites to thrive.
Dust mites are not the only source of proteases found in house dust. For example, proteases from cockroaches are also a source of allergens. Furthermore, it is possible that proteases from cat saliva become airborne as the saliva dries, for example, on the cat's fur. It is likely that such proteases also act as allergens to individuals who are allergic to house dust.
It is known to test house dust in order to determine quantitatively levels of the house dust mite allergen. According to one patent, U.S. Pat. No. 4,806,490, a dust sample is suspended in an aqueous-alcoholic alkali metal hydroxide solution to dissolve or leach out aromatic compounds such as guanine excreted by dust mites, and the resulting solution is mixed with an aromatic diazo compound. A reaction between the aromatic diazo compound and certain excreted aromatic compounds in the solution produces a colour change, with the intensity of the new colour being indicative of the level of excreted proteases in the house dust.